A cutting edge in a cutting tool is exposed to a high temperature environment of about 1000° C. during high-speed processing of a high hardness material. Not only is wear generated by friction and oxidation due to contact with the work piece, but a mechanical shock such as an interruption is also received. Therefore, it is critical for the cutting tool to have an appropriate wear resistance and toughness.
In order to impart such critical wear resistance and toughness to the cutting tool, a hard thin film formed through chemical vapor deposition (hereinafter referred to as ‘CVD’) or physical vapor deposition (hereinafter referred to as ‘PVD’) is typically formed on a surface of a cemented carbide which is used as the cutting tool.
Such a hard thin film is composed of a single layer or multiple layers of a non-oxide based thin film (for example, TiN, TiC, TiCN), an oxide-based thin film having excellent oxidation resistance (for example, Al2O3), or a mixed layer thereof. Examples of the non-oxide based thin film include carbides, nitrides, and carbonitrides of group 4, 5, and 6 metal elements, such as TiN, TiC, and TiCN. Representative examples of the oxide based thin film include α-Al2O3 or κ-Al2O3.
Among these, α-Al2O3 is a stable phase at high temperatures, and thus does not undergo a phase change during the cutting process, but exhibits superb wear resistance. However, for α-Al2O3 to be coated directly onto the non-oxide based thin film, a high temperature of about 1040° C. is required. Here, the α-Al2O3 that is formed has a large crystal grain size (about 1-6 μm), and a large number of defects such as micropores are present in the crystal. Thus, the mechanical strength of the thin film is poor.
In order to overcome such limitations, coating a first stage oxide layer on a non-oxide thin film and then coating α-Al2O3 thereon has been proposed. When such a stage oxide layer is used, the coating temperature may be reduced to about 1000° C. to 1020° C. when coating α-Al2O3. However, the α-Al2O3 prepared through such a method still does not exhibit a sufficient adhesive strength, and thus it is easy for peeling to occur.
As a result, several improved methods for enhancing the adhesive strength between the representative oxide based thin film, that is, α-Al2O3, and the non-oxide based thin film have been proposed.
Japanese Patent Application Laid-open Publication No. 63-195268 discloses a method for coating with a TiCNO layer having a thickness of 5 μm at 1030° C. to 1100° C. and then coating α-Al2O3 thereon at 960° C. to 1000° C. Japanese Patent Application Laid-open Publication No. 02-30406 discloses a method for coating α-Al2O3 on a TiCO or TiCNO layer having a thickness of 1 μm. Japanese Patent Application Laid-open Publication No. 05-345976 discloses a method for using TiCl4, CO, H2, or N2 gas to form a TiCNO or TiCO layer having a thickness of 0.5 to 3 μm and then coating α-Al2O3 thereon at about 1000° C.
Meanwhile, the α-Al2O3 layer has a column-shaped columnar structure. Consequently, since the grain size of the α-Al2O3 layer is large, it is difficult for the surface roughness of the lower layer formed below the α-Al2O3 layer to be sufficiently filled in. As a result, pores are formed in the interfacial region, and thereby there is a tendency for the adhesive strength to be reduced.
Therefore, as disclosed in the prior art patent documents, even when a bonding layer is made of a variety of compositions, it is difficult to achieve a sufficient adhesive strength for the α-Al2O3 layer.